Laser, an acronym, means amplification of amplitude-, frequency- and phase-coherent electromagnetic waves generated by a suitable pumping process inside a closed region composed of a mixture of relevant radiating atoms and molecules, the energy levels of which fully conforms to a stimulated emission created by a feedback of some portion of the coherent electromagnetic wave at the output port of the region.
The areas of use of lasers get very diverse along with the increasing in the developments of the design and manufacturing of high technology products. If a categorization according to priorities of using the highest technology in industrial products is made, it is seen that health and war technology equipments are more dominant over the other industry sectors. The lasers can have continuous wave (CW) mode lasing and/or pulsed-mode lasing and have conspicuous and effective characteristics such as lethal or non lethal effects, physiological, psychological or directly physical effect depending on the energy transferred into any target in modern war and health equipments. In order to make exact and correct evaluations about the resultant effects of any laser source on any target, it seems that it is an unavoidable approach to make spectral power distribution, total power and energy measurements of the relevant laser source in addition to the determination of surface absorption/reflection, structural and atomic/molecular bonding characteristics of the target.
The spectral power distribution (W/nm) and the total power (W) carry a significant meaning for a CW mode/regime laser source because the knowledge of total power of a CW laser is enough to calculate the total exposure over time t (s) for surface of any relevant target in (J) and (J/cm2), energy density, by taking the target absorptiveness into account. Differently from the measurement of total power of CW laser in W, the measurement of laser energy (J) per pulse for a Pulsed Type Laser Source in time domain conveys a significant meaning, because the exposure of the Pulsed Type Laser Source depends on pulse width (PW) and peak power P0 of the Pulsed Type Laser Source, considering surface absorption/reflection, structural and atomic/molecular bonding properties of the target.
NOTE: The term “Chopped Type Laser Source” in the invention means the modulated laser source generated by chopping CW Gaussian Laser Beams of CW Laser Source(s) mechanically by means of the group of the circular and metallic choppers, which is strict a part of FCIS based-LEMCS invented. The term “Pulsed Type Laser Source” in the invention means any other laser source which is different from the “Chopped Type Laser Source”, and which is not a part of FCIS based-LEMCS invented. Nevertheless, both “Chopped Type Laser Source” and “Pulsed Type Laser Source” in the invention produce laser pulses, both of which have Gaussian beam profile, as infinite pulse train in time domain and finally, the terra “Gaussian Laser Beam” used in the invention means diffraction limited—transverse electromagnetic mode having the lowest order (TEM00).
The transferred energy into the target by a laser source regardless of CW or pulsed type results in a temperature increase in limited volume of the target, depending on the heat capacity, mass and the initial temperature of the relevant volume of the target. Detecting the temperature increase of the relevant volume of the target resulted from the energy of the laser source can be made via conventional semiconductor type or metal/metal contact type temperature sensors. To gain signal to noise ratio (SNR) of detection system, which is one of the most important parameter increasing the measurement uncertainty, the separation of the temperature variation caused by energy transfer requires to he extended. The way to extend the separation between the initial temperature and the final temperature caused by laser source energy is to reduce the heat capacity (specific heat) of the target which is accomplished by reducing the initial temperature of the target down to cryogenic level, relying on Bose-Einstein approach. Reducing the initial temperature of the target also minimizes the atomic and molecular vibrations. According to Bose-Einstein statistic for the canonical ensemble, the heat capacity (specific heat) of a solid target reduces exponentially at cryogenic levels of temperature and this physical phenomenon expands the separation between the final and the initial temperature of the target, which expresses an absorbing cavity in a Cryogenic Radiometer (CR) and finally a calorimetric measurement for absolute optical power measurement and also optical energy measurement.
By considering the above summary, the traceable measurements of the laser energy meters and their traceable calibrations can be carried out by measuring the temperature difference (K) between the final and the initial temperature of the target along with inclusion of mass (kg) and the specific heat (J/kg K)), which is a measureable quantity, in the calculations, bearing in mind that: the time constant of the target (or the absorbing cavity). In a CR, the specific heat of the absorbing cavity for the electrical watt (A.V=W) applied within Δt (s) time interval is obtained as a ratio and it is called as thermal coefficient in (W/K), also generating (J/K). In this traceability stage, it is seen that temperature (K), direct current (A) and direct voltage (V) together with traceable time (s) measurement necessary to define the time constant (s) of the target (or the absorbing cavity) and time interval Δt (s) of the electrical power applied to the absorbing cavity should be wholly traceable to primary standards. As a result, the averaged pulse energy of a Pulsed Type Laser Source/Chopped Type Laser Source can be derived by calorimetric methods with traceability of temperature (K), direct current (A), direct voltage (V), and time (s).
Under the illumination of the above briefing related to the traceability chain of optical power and energy, it is understood that we need an optical power measurement in (W) and a time measurement in (s) for realization of the averaged pulse energy (J) of any Pulsed Type Laser Source. The mathematical basis belonging to deriving the averaged pulse energy of the Pulsed Type Laser Source is given by taking the laser pulses having a pulse width of PW (s) and a period of T (s), the peak power of which is P0 (W), as an infinite pulse trainin time domain. Referring to the periodic pulse shape of Pulsed Type Laser Source in the style of an infinite pulse wave train, the function of output power of the Pulsed Type Laser Source for a period of T (s) is defined as P(t) in Eq.(1):
                              P          ⁡                      (            t            )                          =                              {                                                                                P                    0                                                                                        0                    ≤                    t                    ≤                    PW                                                                                                0                                                                      PW                    <                    t                    <                    T                                                                        }                    ⁢                                          ⁢                      (            W            )                                              (        1        )            
And P(t) is a periodical function, as an infinite laser pulse train in time domain, P(t)=P(t+T). Pulse energy of the single pulse of Pulsed Type Laser Source, PE (J);PE=P0.PW(J)  (2)
The average power of the Pulsed Type Laser Source, Pav;
                              P          av                =                              〈                          P              ⁡                              (                t                )                                      〉                    =                                    1              T                        ⁢                                          ∫                0                T                            ⁢                                                P                  ⁡                                      (                    t                    )                                                  ⁢                                                                  ⁢                d                ⁢                                                                  ⁢                t                ⁢                                                                  ⁢                                  (                  W                  )                                                                                        (        3        )            
If the integral is written in the most general form and in the averaged terms by taking the Duty Cycle into account, Eq.(4) is obtained:
                              P          av                =                                            N              ⁢                                                          ⁢                              PW                av                                                    T              av                                ⁢                      P            0                    ⁢                                          ⁢                      (            W            )                                              (        4        )            
                              Duty          ⁢                                          ⁢                      Cycle            av                          =                                                            N                ⁢                                                                  ⁢                                  PW                  av                                                            T                av                                      ⁢                          T              av                                =                                    PW              av                        +                          DT              av                                                          (        5        )            
                              P          av                =                              N                                          T                av                            ⁢                                                                            ⁢                      PE            av                    ⁢                                          ⁢                      (            W            )                                              (        6        )            
Where the averaged pulse width is PW and the averaged dead time is DTav in an averaged repetition period Tav for an infinite laser pulse train generated by Pulsed Type Laser Source. The averaged pulse energy of Pulsed Type Laser Source is obtained by multiplying N with PEav. N is the pulse number and is equal to 1 for periodic and infinite pulse train in time domain,
Eq.(4) and (6) give us a very useful approach to derive the averaged pulse energy PEav of Pulsed Type Laser Source. If repetition period T and the averaged optical power Pav of Pulsed Type Laser Source are measured, the averaged pulse energy can easily be calculated. These measurements of the averaged repetition period Tav and the averaged optical power Pav should be performed traceable to primary level standards, which are 133Cs (or 87Rb) Atomic Frequency Standard in time scale (s), and optical power transfer standard calibrated against absolute optical power measurement system called CR in optical power scale (W) [1 and 2], and an electrometer in direct current scale (A) traceable to Quantum Hall System, and DC Josephson System. The precise measurements of Tav and Pav traceable the primary level standards exhibits a process without measuring the temperature change caused by the averaged pulse energy of a Pulsed Type Laser Source. The most uncertainty contribution of the calorimetric measurements of the averaged pulse energy is resulted from the determination time constant of an absorbing surface (target) and so the pulse and the modulation response of the absorbing cavity (target). In addition to the elimination of time constant of FCIS time/frequency related measurements in the invention, the new configuration of the integrating sphere invented, called as FCIS, enables the user positioning the laser beam having a Gaussian profile on the same optical axis with respect to the entrance port for every calibration process so the reproducibility of the calibration and the measurement processes are increased with the new configuration of FCIS.
Photovoltaic type photodiodes generate an integrated photocurrent as response of the optical flux falling on the sensitive surfaces, corresponding to average optical power of the incident optical flux. This is also valid for the ultra fast photodiodes having very fast impulse response, like positive-intrinsic-negative (PIN) photodiodes as well as avalanched type photodiodes supplied with a reversed voltage bias which reduces the diffusion capacity of the photodiode, still used in optical time domain retlectometer instruments. The integrated photocurrent is also generated for the relatively small portion of light flux within optical pulses having ultra short time intervals, such as Δt≅20×10−12s.
The parameters to be measured to determine the averaged pulse energy PEav of the Pulsed Type Laser Source in Eq.(6) are averaged repetition period Tav, number of pulses N having a varying pulse width PW, and average power Pav, corresponding to an average photocurrent Iav generated by the First Photodiode, which is InGaAs_1 for the apparatus designed as one embodiment in the invention. Eq.(6) can be re-written as Eq.(7) by considering the spectral responsivity of the First Photodiode in order to obtain the averaged pulse energy of Pulsed Type Laser Source in FIG. 1 and FIG. 2.
                              P          av                =                                            I              av                                      R              FCIS              λ                                =                                    N                              T                av                                      ⁢                          PE              av                        ⁢                                                  ⁢                          (              W              )                                                          (        7        )            
Where RFCISλ spectral power responsivity of FCIS, to which the First Photodiode is mounted, in A/W. As stated above, Iav is measured by the First Photodiode placed orthogonally with respect to laser entrance port of FCIS. Iav=Iph(t), Iph(t)=Iph(t+T) is the periodic pulse type photocurrent, generated by P(t). Iav is the time average of Iph(t)=Iph0rect(t). Tav (and/or fav) is measured by using a second photodiode mounted on an internal steel hemisphere, which is placed on directly opposite Gaussian laser beam entrance port of FCIS of FCIS based-LEMCS. For single pulse having a unit amplitude, rect(t) function is defined as in Eq.(8).
                              rect          ⁡                      (            t            )                          =                  {                                                    0                                                              PW                  <                  t                  <                  ∞                                                                                    1                                                              0                  <                  t                  <                  PW                                                              }                                    (        8        )            
This definition of a single pulse given in Eq.(8) will be useful for the description of the pulse response of the First Photodiode and for the description of use of a second photodiode, which is different from the First Photodiode, and which has a relatively small time constant, to carry out time/frequency related measurements in Eq.(7). RFCISλ in Eq.(7) is obtained by calibrating FCIS based-LEMCS against the Optical Power Transfer Standard, which is an InGaAs based spectralon sphere radiometer directly and which is absolutely calibrated against Cryogenic Radiometer (CR) in this invention. Another alternative process of deriving the RFCISλ of the First Photodiode can be performed with a relatively higher uncertainty arising from the surface non uniformity by referencing a flat spectral response Electrically Calibrated Pyroelectric Radiometer (ECPR), traceable to CR, in such a way that the whole spectra of 900 nm to 1650 nm of the First Photodiode is covered.
NOTE: The use of different type of Optical Power Transfer Standard doesn't disturb the philosophy of the invention because FCIS based-LEMCS is one embodiment.
According to Eq.(7), if Iav, Tav, and RFCISλ are measured, the specified and averaged pulse energy PEavof the Pulsed Type Laser Source can be calculated with an expanded uncertainty by taking the related partial derivations of Iav, Tav, and RFCISλ into the calculations.
The Second Photodiode, which is InGaAs_2 in the invention as one embodiment, is assembled with a first multimode (MM) patch cord. FC/PC connector end of the first multiniode (MM) patch cord is combined to a Mechanical Attenuator and the HMS connector end of the first MM fiber patch cord having a Zr ferrule is mounted on the center of the inner wall of an internal steel hemisphere, which is placed inside FCIS, which has a smaller diameter than that of FCIS. The Second Photodiode combined with the hemisphere through a second MM patch cord, the Mechanical Attenuator, and the first MM patch cord having ceramic and Zr ferrules is used for the time measurements such as averaged repetition period Tav and averaged repetition frequency fav in Eq.(7), cutoff limit is 6 GHz. The second use purpose of the Second Photodiode is to coincide optical axes of FCIS and Pulsed Type Laser Source, Chopped Type Laser Source, and CW Laser Source. The Internal Steel Hemisphere is made from stainless steel and is assembled with a Zr ferrule of the first MM optical fiber patch cord. The internal Steel Hemisphere is so settled inside FCIS that Gaussian laser beam entrance port of FCIS of FCIS based-LEMCS sees directly the center of the Internal Steel Hemisphere, at the center of which Zr ferrule of HMS connector end of the first MM optical fiber patch cord is mounted back 0.2 mm from the inner surface. The placement of a internal steel hemisphere together with Zr ferrule of HMS connector end of the first MM optical fiber patch cord is one of the important points of this invention.
The practical way to search the frequency response of any electronic device, such as a pin photodiode in this invention, is to apply a pulse having a varying pulse width and a varying period to the electronic device. According to the Fourier transformation between time and frequency domains, as long as the pulse width PW is made relatively narrow, it is seen that the frequency content of the pulse increases. As a result, an ideal δ(t)-impulse function in time domain covers a frequency range from zero to infinite theoretically. The periodic optical pulses P(t) generated by the Pulsed Type Laser Source, the pulse width PW of which are adjustable, can be defined as a sum of odd (sinus) harmonics in Fourier series, and they have the decreasing amplitude with a DC component, the period of which is T (s), matching the repetition frequency f (Hz). Correspondingly, the modulation frequency response of FCIS is obtained the sum of all the responses of FCIS through the First Photodiode against the each frequency component obtained from the Fourier series. When the frequency content of Fourier Series of a periodic pulse train repeated within repetition period T is seen, the first term, which has the highest amplitude, is f (Hz), which is exactly the same as the repetition frequency of the Pulsed Type Laser Source. The successive frequency terms of sinus are lined up to 2f, 3f, 4f, . . . , nf, where n is the number of the summed frequency components, with the decreasing amplitude. It should be noted that making the pulse width PW in time domain be narrow increases the frequency contents. Therefore the pulse response characteristics and the modulation frequency response characteristics of the First Photodiode of FCIS, which is used to measure the averaged photocurrent Iav proportional to the averaged optical power Pav, are presented together herein. It is pointed out that. FCIS based-LEMCS and the method described in the invention can operate up to a repetition rate of 1 MHz which is the cutoff limit of the First Photodiode. In order to use FCIS based-LEMCS correctly and properly in measuring the average optical power Pav, FCIS based-LEMS should be held within the frequency range in which the First Photodiode of FCIS based-LEMCS has a flat frequency response. If the repetition frequency is too high the First. Photodiode to catch, which corresponds to being too faster rising and falling edge times, and too narrower pulse widths and dead times, it is impossible to convert the average optical power of such an infinite pulse train of Pulsed Type Laser Source having a peak power of P0 into the average photocurrent. This is an inherent behavior for the photodiodes as well as the electronic circuit exhibiting low pass filter behavior.
The First Photodiode behaves as a RC low pass filter for the increasing modulation frequencies resulted from the equivalent circuits composed of the total of junction capacitance (Cj) and stray capacitance (Cs) of the First Photodiode, which acts as in reversed bias condition when light flux falls onto the sensitive surface of the First Photodiode. Correspondingly, diffusion capacity of the First Photodiode, which describes the rearrangement of the minority carriers within the depletion region under the forward bias, is not considered in this equivalent circuit. The equivalent circuit of the First Photodiode in FCIS of FCIS based-LEMCS is shown in FIG. 3. Resultantly, the equivalent capacitance is Ceq=Cj+Cs≅200 pF, and at zero bias, Cj≅20 pF at 25° C. The equivalent resistance of the First Photodiode consists of parallel shunt resistance (Rsh), serial resistance of bulk semiconductor (Rs), and parallel input resistance of the following current to voltage amplifier (Ri), directly corresponding to the electrometer used in this invention. The equivalent resistance is 1/Req=(1/Rsh+1/(Rs+Ri)). For the First Photodiode used in the invention, Rsh≅10 MΩ, Rs≅800 Ω and Ri≅0.72 Ω, yields an equivalent resistance Req≅800 Ω, corresponding to a time constant of Req Ceq≅16×10−8 s (160 ns) for the First Photodiode at 25° C. Due to the fact that any additional reversed bias voltage is not applied to the First Photodiode, the photocurrent Iph(t) doesn't contain dark current and it contains the photocurrent induced by the average power of Pulsed Type Laser Source which has Poisson type noise distribution and Boltzmann Noise current. Even if not applying any reversed bias to the First Photodiode in the invention reduces the higher frequency limit, the noise limit of the First Photodiode of FCIS become better and this approach enables FCIS reaching a threshold level of 1 nA in non-cooling mode, corresponding to 16.5 pJ at 1550 nm level for a Duty Cycle of 0.17 at 1 MHz, −3 dB frequency range, in practice.
In this section the pulse and the modulation frequency responses of FCIS based-LEMCS invented: Modulation frequency response of FCIS caused by the RC low pass filter type equivalent circuit consisting from the resistance and capacitance values of the First Photodiode, other effect restricting the pulse and the modulation frequency responses of FCIS is the time constant (τ) of FCIS, based on the diameter of the integrating sphere, coating average reflectance of the inner coating, and light velocity. The time constant (τ) of FCIS is an effective component on determination of average power Pav and resultantly averaged pulse energy by FCIS through the First Photodiode.
By considering the below evaluations concerning with the modulation frequency response of FCIS through the First Photodiode against the rising, the falling edges of the optical light pulses together with pulse width PW, generated by Pulsed Type Laser Source, the pulse response of FCIS should be taken into account, because repetition rate of 1 MHz, corresponding to a period of 1 μs, should have the rise and the fall times relatively very lower than 1 μs. For these edges together with relatively short PW can be regarded as δ-delta impulse function for FCIS with an inner diameter of 15 cm which has the First Photodiode and the investigation is made according to the modulation frequency response pertaining to the repetition frequencies up to 1 MHz. As a result, it is obvious that increasing of the modulation frequency gives rise to shortening the rise and the fall time of the pulses as well as PW. In this case, the pulse energy term in Eq.(7) should contain the pulse response term. Therefore Eq.(7) can be rearranged and considered in two parts as in Eq.(9) and as in Eq.(10). First, the pulse response needing to be investigated for measuring Pav in the invention is that of the First Photodiode, behaving as a RC low pass filter against the optical pulses having increasing repetition rates, If the complete pulse response of a RC low pass filter circuit composed of the parallel combination of Req and Ceq is calculated, the rise time and the fall time along with PW at the output photocurrent Iav of the First Photodiode also exhibits exponential behavior. In this case, by assuming the laser pulse entering in FCIS, the peak power of P0 can be written as Eq.(9) for single laser pulse, containing the pulse response of FCIS and the pulse response of the First Photodiode, and it should be noted that Iph0 should have a rectangular function form.
                              P          0                =                                                            I                pho                                            R                FCIS                λ                                      ⁢                          ζ                              pd_                ⁢                1                                      ⁢                          ζ              FCIS                        ⁢                                                  ⁢            0                    <          t          <                      PW            ⁢                                                  ⁢                          (              W              )                                                          (        9        )            
Where
      ζ    FCIS    =      (          1      -              e                  -                                                    t                r                            +              PW              +                              t                f                                      r                                )  is the pulse response of FCIS against the laser pulse and ζpd_1 is the pulse response function of the First Photodiode of FCIS, respectively. A pulse can be divided into three parts. The first part is rising edge tr, the second part is pulse width PW, and the third part is falling edge, tf. However, in the characterization of the pulse response of the First Photodiode, to think an integrated and complete part of the response of the First Photodiode against the rising edge and the pulse width of the pulse is correct, because in these parts of time of the single pulse, the capacitors of the equivalent circuit are the state of charging and keeping stable. The third part of the single pulse directly corresponds to discharging the capacitors and so third part of the pulse should be represented by a different function. The pulse response function ζpd_1, which is composed of the summation the responses written for three pulse parts, directly relies on the time of charging of capacitors and discharging capacitors through relevant equivalent resistances. This analysis can easily be made by using a continuous convolution of the single pulse with the equivalent circuit of the First Photodiode.
                              ζ                      pd_            ⁢            1                          =                  {                                                                                          (                                          1                      -                                              e                                                  -                                                      t                                                          16                              ×                                                              10                                                                  -                                  8                                                                                                                                                                                                          )                                    ⁢                                      rect                    ⁡                                          (                      t                      )                                                                                                                    t                  =                                                            t                      r                                        +                    PW                                                                                                0                  ≤                  t                  ≤                  PW                                                                                                                          (                                                                  e                                                  -                                                                                    (                                                              t                                -                                PW                                                            )                                                                                      16                              ×                                                              10                                                                  -                                  8                                                                                                                                                                                        -                      1                                        )                                    ⁢                                      rect                    ⁡                                          (                                              t                        -                        PW                                            )                                                                                                                    t                  =                                      t                    f                                                                                                ∞                  >                  t                  >                  PW                                                              }                                    (        10        )            
Where ζpd_1 is the multiplier for Iph0, which matches the initial voltage on Ceq just before the discharging of the equivalent capacitor Ceq was started for ∞t>t>PW. The pulse energy PE0 of a single laser pulse including the pulse responses is,
                              PE          0                =                              PW            ⁢                                                  ⁢                          P              0                                =                      PW            ⁢                                          I                pho                                            R                FCIS                λ                                      ⁢                          ζ                              pd_                ⁢                1                                      ⁢                          ζ              FCIS                        ⁢                                                  ⁢                          (              W              )                                                          (        11        )            Where tr, tf, and PW are the rise time, the fall time and the pulse width of the pulse of the laser pulse. For the single pulse PW>>160 ns, and tr<<PW for both pulse response functions;
            ζ              pd_        ⁢        1              ≅          (              1        -                  e                      -                          PW                              (                                                      R                    eq                                    ⁢                                      C                    eq                                                  )                                                        )        →  1and PW=4.6 (ReqCeq)≅736 ns. The pulse width of 736 ns is sufficiently larger than 160 ns for this approximation, producing 0.99 Iav.
The parameter
  τ  =            -              2        3              ⁢          D      c        ⁢          1              ln        ⁡                  (          ρ          )                    is the time constant of FCIS, ρ is the average reflectance of the inner coating of FCIS, D is diameter of FCIS, and c is the velocity of light in vacuum. The term
  1      ln    ⁡          (      ρ      )      corresponds to average number of reflections until a photon is to be absorbed [3 and 4]. It is possible to measure of FCIS by measuring the rise times of a very short pulse, which has a pulse width of a few ps, at the entrance port and at the detector port after first reflection. Regarding the time constant τ of FCIS, bearing in mind that quasi-exponential absorption behavior of the inner wall coating of FCIS having highly diffusive reflection is in accordance with the Beer Lambert Law for a photon flux emitted from Pulsed Type Laser Source and assuming that the inner coating of FCIS is nearly uniform and the inner volume of FCIS having a diameter of 15 cm is nearly isotropic, we can say that the pulse response of integrating sphere have an exponential behaviors for rise and fall times of the pulse of the Gaussian Laser Beam due to the time constant (τ) and the dissipation of diffusely reflected irradiance of a single light pulse on the entire inner surface of FCIS reaches to any point within an elapsed time Δt′ inside of FCIS [3 and 4]. According to the above assessments, if PW is larger than τ and for CW laser beam instead of pulse Pav goes to P0. If PW is smaller than τ, corresponding to ultra short pulse condition, there is no sufficient time for the uniform and diffuse reflection of a single pulse inside FCIS and Pav cannot be detected. One of the important points to determine the pulse and the modulation frequency response of the First Photodiode used in the application of measuring the average power of the Pulsed Type Laser Source in the invention is to characterize how many portion of Gaussian Laser Beam entering FCIS is diffusely reflected inside FCIS. For this characterization, the ratio between the diffuse power inside FCIS and the direct power entering in FCIS directly corresponds to
            η      diffuse        =                  P        0        diff                    P        0              ,which is the power efficiency between the diffuse power inside FCIS and the direct power entering in the FCIS, f−3dBFCIS=1/(2πτ) is the cutoff frequency of FCIS. The direct spectral responsivity calibration of FCIS based LEMS against Optical Power Transfer Standard, which will be described in the section “Determination of the spectral responsivity RFCISλ of FCIS based-LEMCS”, eliminates ηdiffuse in Eq.(12) because RFCISλ (A/W) is obtained from the optical flux diffusely reflected inside FCIS and ηdiffuse in RFCISλ is at the denominator in Eq.(12). The time constant of FCIS in the invention is τ≅3 ns, corresponding to f−3dBFCIS≅53 MHz, for a wall coating having an average value of 0.90. In the pulse response function of FCIS based-LEMCS, the pulse response of FCIS based-LEMCS comprises two parts given in Eq.(12). The first part is related to the geometric characteristics of FCIS of FCIS based-LEMCS together with its inner coating property and the second part is related to the equivalent circuit of the First Photodiode. By comparing Eq.(10) and Eq.(11), Eq.(12) is written as a complete and final equation.
                              PE          0                =                  PW          ⁢                                    I              pho                                      R              FCIS              λ                                ⁢                      (                          1              -                              e                                  -                                      PW                                          3                      ×                                              10                                                  -                          9                                                                                                                                          )                    ⁢                      ζ                          pd_              ⁢              1                                ⁢                                          ⁢                      (            J            )                                              (        12        )            
In Eq.(12), it is seen that this type of pulse response function ζpd_1 of the First Photodiode causes the distortion of the ideal pulse shape of photocurrent Iph0 generated by the single laser pulse, depending on time constant of the equivalent circuit171 of the First Photodiode, ReqCeq. This shape distortion, is especially resulted from the relatively larger time constant of the First Photodiode ReqCeq=160 ns, rather than time constant of FCIS τ≅3 ns. The distortion occurs also in phase of the photocurrent pulse produced by the laser pulse with respect to the laser pulse. These distortions negatively affect to carry out the time/frequency related measurements by means of the First Photodiode. These distortions are characterized in FIG. 2 as PW′ and DT′ for the photocurrent Iph(t) which is generated by the First Photodiode against Pulse Width and Dead Time of Pulsed/Chopped Gaussian laser beams of Pulsed Type Laser Source and Chopped Type Laser Source. To defeat the problematic condition resulted from the distortion based on unreliable time/frequency related measurements, a second photodiode having a relatively higher low cutoff frequency is placed and reserved in the invention, which is one of the new implementations presented in the invention. The averaged photocurrent measurements and time/frequency related measurements are carried out separately by different photodiodes, called the First Photodiode and called the second in the invention.
The term of Eq.(12)
      ζ          pd_      ⁢      1        ≅      (          1      -              e                  -                      PW                          (                              16                ×                                  10                                      -                    8                                                              )                                            )  for the single pulse PW>>160 ns, and PW>>tr, tf, which is the pulse response of the First Photodiode mounted to FCIS in Eq.(12), is an effective parameter for the relatively short pulse widths at the higher modulation frequencies, PW of which approaches 736 ns or shorter. A pulse width PW of 736 ns forms the upper time limit for the First Photodiode of FCIS in the invention together with sufficient and necessary Dead Time DT for heat dissipation, which is detailed in the section of “DESCRIPTION”. In case of using any other photodiode having ReCeq lower than 160 ns instead of the First Photodiode, to obtain a new PW narrower than 0.736 μs is obvious. At same time, this is also valid for the term of Eq.(12).
            ζ      FCIS        =                  ⁢          (              1        -                  e                      -                          PW                              3                ×                                  10                                      -                    9                                                                                          )        ,which is the pulse response of FCIS of FCIS based-LEMCS in Eq.(12). The width of the laser pulses having PW wider than 4.6 τ≅14 ns is sufficient to allow peak power P0 of 0.99 to dissipate (spread) in the inner surface of FCIS. Due to the fact that both of the First Photodiode and the FCIS behave as a low pass filter, provided that the pulse width PW of Pulsed Type Laser Source is sufficiently wide, the peak pulse energy of the infinite laser pulse train is correctly measured. If the pulse width of Pulsed Type Laser Source is very short, relative to pulse response characteristics of FCIS and the First Photodiode, the rise and the fall times of infinite laser pulses of Pulsed Type Laser Source is retarded by low pass filter characteristics of the First Photodiode and the rise and fall times have slower slopes than original states. As a result this retarded rise and fall times causes to carry out measurement of averaged repetition period Tav (or averaged repetition frequency fav) having low precision which corresponds to high measurement uncertainty in time/frequency related measurements by using the output photocurrent Iph(t) of the First Photodiode. And the pulse width PW and the dead time DT values of infinite laser pulse train of Pulsed Type Laser Source are sensed and converted as PW′ and DT′ as in FIG. 3. In order to defeat this problematic condition due to limited pulse response of the First Photodiode, in the invention, the time frequency related measurements are carried out by second photodiode. FCIS based-LEMCS is one embodiment and the variation in numerical values doesn't change the philosophy of the invention.
The two of the most related international patents still in progress to the invention described herein are introduced at the following:
The invention described in US2013250997 (A1) deals with the thermopile type laser energy conversion. The thermopile theory of detecting the laser pulse energy relies on the temperature drop between the hot and cold thermocouple junctions across which the heat, caused by laser energy, flows radially, and the temperature drop results in a voltage output proportional to laser energy applied. This voltage output proportional to laser energy is collected with an integrating circuit receiving the electrical output from the thermopile, such that the energy of at least one pulse of the beam can be determined by integrating over time the electrical output arising from the at least one pulse. The response time of such a thermopile sensor is typically no faster than 1 s for reaching 95% of the final reading and the maximum repetition period to be measured with this system was stated as 10 Hz. However, FCIS based-LEMCS doesn't contain any thermopile type temperature sensor. Instead of using a thermopile, FCIS based-LEMCS is mainly composed of newly configured integrating sphere assembled with the photovoltaic type photodiodes, called the First Photodiode and the Second Photodiode and the averaged pulse energy of the Pulsed Type Laser Source e is determine by measuring by the averaged photocurrent proportional to the peak power of the Pulsed Type Laser Source and by measuring time related measurements of the Pulsed Type Laser Source for a repetition frequency extending to 1 MHz, corresponding to a repetition period of 1 μs, which is relatively very higher response time with respect to the system described in US2013250997 (A1). FCIS based-LEMCS described herein is one embodiment, the upper cutoff frequencies of the First Photodiode and the Second Photodiode don't disturb the philosophy of the invention described herein and so the photodiodes, the cutoff frequencies of which are higher than 1 MHz and 6 GHz, really and undoubtedly get better. Additionally, both the First Photodiode and the Second Photodiode specified herein can be exchanged with different types of semiconductor detector depending on the spectral power distribution of the laser to be engaged in the application
Another invention described in JPS63100335(A) deals with securely detecting the energy of a laser beam by providing a laser detector for detecting the energy of a laser beam which is reflected and uniformed by a laser beam scattering device, which is a motorized chopper, and an integrating sphere. The detector mounted to the integrating sphere in JPS63100335(A) senses the uniformly scattered and reflected laser beam portion and the invented systems acts as laser energy presence sensor. Any pulse energy measurement procedure of laser is not seen in JPS63100335(A). However, beyond the detection of presence of laser energy, FCIS based-LEMCS described herein provides both the measurement capability of the averaged pulse energy of the Pulsed Type Laser Source and the calibration of Commercial Laser Energy Meter against FCIS based-LEMCS by using Chopped Type Laser Source, which is a part of FCIS based-LEMCS, and which is traceable to primary level standards.